1. Field of the Invention
The present invention relates to a radio wave receiving device.
2. Description of Prior Art
Currently, a long-wave standard wave including time data or time code is transmitted in various countries (for example, Japan, America, Germany and the like). In Japan, the long-wave standard waves of 40 kHz and 60 kHz that have been subjected to amplitude modulation by the time code are transmitted from two transmitting stations in Hukushima prefecture and Saga prefecture.
A watch so-called radio wave watch which corrects the time data of a timekeeping circuit has been put to practical use as a radio wave receiving device to receive this standard wave. The radio wave watch of this kind cannot receive the standard wave in a building in which the radio wave is hard to reach, so that the time may not be corrected. As a technique to correct the time of the radio wave watch in an environment where it is difficult to receive the standard wave, the technique to relay the standard wave has been known, and a repeater has been also known as a device for this technique.
The repeater receives the standard wave, and transmits the time data included in the standard wave through a relayed radio wave.
An example of the repeater includes one which transmits the time date included in the received standard wave by infrared radiation.
The standard wave is transmitted from the two transmitting stations with different frequencies in Japan. Therefore, a radio wave watch and a repeater which can receive the standard wave of both frequencies of 40 kHz and 60 kHz, that is, the radio wave watch and the repeater which was multibanded have been known. Specifically, there is one which selectively receives one of the standard waves of two frequencies, and converts the received standard wave into a relayed radio wave to transmit it.
However, in the above case, the frequency of the relayed radio wave transmitted by the repeater is the same as the frequency of the standard wave, so that the relayed radio wave is superposed on the standard wave transmitted from the transmitting station. Thus, if the standard wave and the replayed radio wave are out of phase, the original standard wave may be damaged, thereby interfering with the reception of the original standard wave.
For solving the problems, considered is a structure in which a repeater transmits a time data included in a received standard wave to a radio wave watch through a replayed radio wave of infrared radiation, and the radio wave watch can receive both of the standard wave transmitted from the transmitting station and the relayed radio wave of the infrared radiation transmitted from the repeater. With this structure, since the relayed radio wave is the infrared radiation, the relayed radio wave is not superposed on the standard wave.
However, in this case, the radio wave watch needs to comprise both of a receiving circuit for the standard wave and a receiving circuit for the infrared radiation. Therefore, two systems of the receiving circuits with different reception frequencies are provided, which increase the size of the circuit of the radio wave watch.
Meanwhile, considered is a radio wave watch of super-heterodyne system in which the frequency of the relayed radio wave is set to low frequency, and the standard wave and the relayed radio wave are selectively received. However, in this system, a reception signal and a local oscillation signal are synthesized to be converted into an intermediate-frequency signal having a predetermined frequency, so that the frequency of the local oscillation signal needs to be changed according to a reception frequency.